It is well known that for the color development of a silver halide photographic material, after exposure to light, an oxidized aromatic primary amine developing agent can be reacted with a dye forming coupler to form a color image. In this procedure, the subtractive color process is ordinarily used for color reproduction, and in order to reproduce blue, green and red color images of yellow, magenta and cyan are formed. The colors of the color images are respectively the complementary colors of blue, green and red.
Couplers of the pyrazolone, pyrazolotriazole, pyrazolobenzimidazole, cyanoacetophenone or indazolone type are generally used for forming magenta color images.
It is required in a multilayer color photographic light-sensitive material that couplers are fixed in respective layers separated from each other in order to reduce color mixing and improve color reproduction.
One method for rendering a coupler diffusion-resistant is to utilize a polymer coupler obtained by polymerization of a monomer coupler. As a method of adding a polymer coupler in a latex form to a hydrophilic colloid composition, a method in which a latex prepared by an emulsion polymerization method is directly added to a gelatine silver halide emulsion and a method in which an oleophilic polymer coupler obtained by polymerization of a monomer coupler is dispersed in a latex form in an aqueous gelatin solution have been heretofore known. Some examples of the former emulsion polymerization methods include an emulsion polymerization method in an aqueous gelatin phase as described in U.S Pat. No. 3,370,952 and an emulsion polymerization method in water as described in U.S. Pat. No. 4,080,211. An example of the latter method in which a oleophilic polymer coupler is dispersed in a latex form in gelatin is described in U.S. Pat. No. 3,451,820.
The method of adding a polymer coupler in a latex form to a hydrophilic colloid composition has many advantages in comparison with other methods. First of all, the deterioration of strength of the film formed is prevented, because the hydrophobic substance is in a latex form. Also, since the latex can contain coupler monomers in a high concentration, it is easy to incorporate couplers in a high concentration into a photographic emulsion, and the increase of viscosity is small. Furthermore, color mixing is prevented, since a polymer coupler is completely immobilized and the precipitation of couplers in the emulsion layer is small.
The addition of magenta polymer couplers in a latex form to a gelatino silver halide emulsion, are described, for example, in U.S. Pat. No. 4,080,211, British Pat. No. 1,247,688, U.S. Pat. Nos. 3,451,802 and 3,926,436 and West German Pat. No. 2,725,591, etc.
However, these polymer coupler latexes have a number of problems which need to be eliminated, in addition to the many advantages described above. The problems include the following:
1. The rate of the coupling reaction is poor and thus the density of dye formed is very low.
2. Undesirable fog is readily formed upon color development.
3. The fastness to humidity and heat of the color image is inferior.
4. The resistivity to formalin is very poor. The term "resistivity to formalin" as used herein means a resistivity to degradation in the color forming property of couplers due to formalin gas generated from furniture, etc.
More particularly, they have major disadvantages in coupling reactivity and resistivity to formalin. With respect to the coupling reactivity, no improvement is obtained by using the two-equivalent magenta polymer coupler latex described in West German Pat. No. 2,725,591 and U.S. Pat. No. 3,926,436. Although some degree of improvement in the color forming property is achieved with a two-equivalent magenta polymer coupler latex as described in Japanese patent application (OPI) No. 94752/82 (the term "OPI" as used herein means an "unexamined published application"), the color forming property is generally low in comparison with low molecular weight two-equivalent magenta couplers and they are apt to form undesirable fog upon color development when preserved under conditions of high temperature or of high temperature and high humidity.